rus
Issue #4/2016
A.Kondrashin, A.Lyamin, V.Sleptsov
4D-technologies for production of three-dimensional integrated circuits
4D-technologies for production of three-dimensional integrated circuits
Main types, the possibilities, advantages and disadvantages of liquid and gas-phase (dry) 3D MID technologies are considered.
Liquid technologies (LS-3D-MID) are the largest class of surface 3D MID-technologies.
2. Liquid S-3D MID
Liquid technologies are divided into the following types:
•two step or two shot molding;
•laser direct structuring (LDS);
•aerosol jet or maskless technology (mesoscale, material deposition – M3D);
•jet sputtering;
•3D photolithography.
2.1. Two step or two shot molding
In the two step molding two different types of polymers are used, one of which may be conductive and the second – dielectric. The first polymer is the main material of the hull structure (first injection), and the second polymer is a filler that selectively fills the free volume and forms the final shape of the part (second injection).
A quite large number of variations of this technology is known, however, the most widely it is developed by Kollmorgen (Germany) in the PCK- and PA-processes [1–4], which are "positive" and "negative" methods of the two step molding by analogy with the processes of conventional photolithography (Fig.1).
More expensive catalytic SKW-process [4] (Fig.2), which is a kind of PCK, is used much less.
As an example, consider the three stages of the PCK-process:
•the conductive substrate 1 (Fig.1a) is made of plastic with a metal filling (e.g., palladium particles) by a injection molding (first injection);
•the second (dielectric) layer of the polymer material 2 that filling the entire surface of the substrate, except areas for the future contact pads, is selectively applied to the surface of the substrate 1 by the second injection in a special mold. The material of the second injection, as a mask, generates the topology of the switching conductors and contact pads (Fig.1b);
•layers of different metals (copper, nickel, gold, etc.) are applied to all open areas of the conductive substrate of the first injection by galvanic (electrochemical) method. Thickness of layers 3 is adjusted by the deposition conditions (Fig.1c).
The key advantages of two step molding:
•structure of conductor paths is formed in the molding process;
•it is possible to apply layers of materials up to 0.5–5 µm;
•high performance (including the high growth rate of metal conductors).
The key disadvantages of the technology:
•the use of two injections of polymers (metallized and not metallized) causes a significant restrictions in the choice of materials, one of which is essentially a mask;
•low resolution (minimum to 0.25 mm, depending on the yield strength of the materials [4]);
•inability to make quick changes in the design of the conductive circuit;
•due to the high cost of the tools, the technology is efficient only under high volume production;
•use of environmentally harmful electrolytes.
2.2. Laser direct structuring
Laser direct structuring (LDS) is an additive method of applying 3D conductive circuits on plastic case [5]. In literature LDS is subdivided into composite and printing-additive.
The basic idea of composite technology of laser direct structuring is the introduction of special metal-containing organic additives in the polymer melt during molding. The additive is heat resistant and prevents the formation of active centers in the process of injection molding. It has a little effect on the basic properties of the plastic, as it contains inactive chemicals that are activated only by local high temperature heating by laser radiation and appear only during ablation of a solid surface. The thermal degradation under the influence of laser radiation on the surface of these composites leads to the evaporation or decomposition of the organic part of the polymer (formation of the rough surface) and to the connection of the individual metal particles in the microspheres, which later become the centers of crystallization of copper during its electrochemical deposition.
Thus, the composite LDS involves four basic operations: injection molding of the substrate (body) with special additives; laser structuring (laser processing of the surface of the substrate with the formation of active centers); metallization (galvanic electrochemical deposition of copper) of modified plastic; the application of the top (protective) layer (except areas of contact pads).
Fig.3. shows the scheme of device for composite laser structuring. The initial plastic substrate 5 is subjected to laser structuring. The rotary mirrors 3 are used for deflection of the laser beam 2. As a result of processing the modified plastic is divided into a metal and organic residual groups forming an activated areas 4. These areas in the future become a center of crystallization during chemical deposition of conductive layers 6 (copper, nickel, gold, etc.) with a total thickness of 10–20 µm.
Printing-additive LDS (Pro Paint LDS) is largely similar to composite with two main differences: first, no metal-containing organic additives in the polymer substrate ("unprepared" plastics are used); second, after the laser structuring of the surface a special conductive "ink" is applied.
Printing-additive LDS consists of six main operations (Fig.4):
•injection molding of the substrate (case);
•laser structuring (processing of the substrate surface by laser light);
•jet-spray application of a conductive "ink", which is a two-component system of "primer" and hardening agent;
•the removal (washing) of the excess "ink" from unstructured surfaces;
•metallization (galvanic electrochemical deposition of copper) of modified plastic;
•application of the top (protective) layer (except areas of contact pads).
For printing – typically the jet-spray application of a conductive "ink" – is used standard sprayer or specialized system (see 2.3). The layer of "ink" with a thickness of about 30 to 40 µm is applied in two stages, and then hardens in the air or using additional heat exposure.
Printing-additive LDS is more labour-intensive and primarily intended for prototyping, since the cost and properties of such products for continued use are not comparable with the properties of composite plastics [6].
The key advantages of LDS:
•only one material is used for forming the substrate (in contrast to two-stage molding);
•flexibility of the process allows to make changes in the design of conductive circuit by reprogramming of the laser;
•resolution to 80 µm, with the possibility of coating in a fairly remote places of the substrate;
•no need to use masks;
•sufficient productivity at relatively low cost of equipment.
Composite technology is also characterized by high adhesion of the deposited metal to the substrate caused by the formation of microrelief in the process of laser activation, and printing-additive technology – by the use of almost any polymer materials and products based on them.
The key disadvantages of the LDS:
•the complexity of the formation of thick layers of functional materials;
•possible thermal damage to the substrate during its laser activation;
•use of environmentally harmful electrolytes.
Additional disadvantage of composite LDS is the limited choice of materials because of the need for the use of special polymers or additives in the manufacture of the substrate, and disadvantage of printing-additive technology is a low adhesion of the conductive "ink" to the developed microrelief obtained by laser activation of the surface, and the presence of additional operations, which significantly increases the cost of manufacturing process.
2.3. Laser subtractive structuring
Laser subtractive structuring (LSS) is based on one of the first, and, consequently, proven technologies of production of circuit boards consisting of a dielectric substrate and foil-coated metal material (mostly copper). A distinctive feature of the LSS from traditional subtractive technology is the impossibility of forming a conductive thick film coating by hot stamping, as on the 2D surface.
Laser subtractive structuring comprises the following steps:
•single- or multicomponent injection molding of the substrate (case);
•deposition of a sublayer of copper (chemical, plasma-chemical, vacuum, thermal, etc.) (Fig.5b). If only chemical deposition of an copper underlayer is used, then before metallization it is advisable to activate the required surface area of the substrate, for example by laser structuring (Fig.5a);
•electrochemical deposition of copper on the metallized surface of the sublayer (Fig.5c);
•deposition of mask (a thin layer of plating resist). A dielectric or conductive material with a high degree of etching selectivity relative to the layer of copper can be used as plating resist (Fig.5d);
•laser structuring (burning) of a thin layer of plating resist (Fig.5e);
•chemical or plasma-chemical etching of copper (Fig.5f);
•if necessary (depending on the operating conditions of the products or other requirements), the removal of residual plating resist (Fig.5g).
2.4. Aerosol-jet deposition
Aerosol-jet deposition [7–8] provides the direct application of conductive films of different liquid materials (suspensions) to the surface of finished 3D parts produced, for example, by injection molding. The suspension includes a solvent, stabilizers, modifiers, polymers, solid particles, etc., and are divided into suspensions of conductive materials, suspensions of dielectric materials and suspensions of semiconductor materials, therefore, the diameter of solid particles may vary considerably depending on the type of deposited material and generally is in the range of 0.05–1 µm.
The device for of aerosol-jet deposition (Fig.6) is based on the special two-section sprayer. In the first section of sprayer the prepared suspension 2 is placed in the evaporator (heater) 1, via which the stream of transport inert gas is passed, creating an aerosol 3 (suspended droplets). Large drops (5 µm or more) return under the action of gravity back into the tank.
The second section of the sprayer is a dual spray nozzle 4. The protective (focusing) gas, which forms the focusing ring of the aerodynamic flow, is supplied under pressure to the outer contour of the spray nozzle. This flow creates in the inner contour of the spray nozzle low pressure, which directs the aerosol due to the pressure difference. The annular aerodynamic flow focusing a stream of aerosol into a narrow beam (flux) with a diameter of 5 to 10 µm, which gives the opportunity to apply the material 6 on the hard-to-reach surfaces 7 with high accuracy and without the use of masks. Distance from spray nozzle to the deposited surface can be up to 5–7 mm.
After application to a substrate, the suspension is cured during the process of sintering or polymerization in the standard drying ovens, ovens for reflow soldering, by laser radiation, LEDs, etc.
The application of aerosol is carried out either by moving the nozzle relative to the substrate or by moving the substrate relative to the nozzle. To obtain separate structures (tracks, pads, or elements of TEU) flow of aerosol can be damped by special device.
The key advantages of the technology:
•only one material is used in the process of forming the substrate;
•it is possible to apply thin layers (10–20 nm) of materials with a resolution down to 5 µm;
•avoidance of thermal damage of the substrate;
•flexibility of the process that allows to quickly make changes to the pattern of the conductive circuits;
•the possibility of obtaining conductive, semiconducting, dielectric, and polymer (PTF, polymer precision thick-film) coatings on virtually all substrate materials;
•no need to use masks;
•possibility of applying coatings in hard-to-reach areas of the substrate;
•high performance at a relatively low cost of processing equipment.
The key disadvantages of the technology:
•use of two gases (transport and focusing);
•need for careful selection of the suspensions for various materials of the substrate to ensure good adhesion of the deposited material;
•need for the curing.
2.5. Jet sputtering (metallization)
Jet sputtering is known for a long time and currently it is widely used in printing, in the production of packaging materials, for applying a photoresist in microelectronics, in printed electronics, etc.
At jet metallization, the scheme of which is shown in Fig.7, the applied material is deposited on free surfaces of the 3D substrate 3 in the form of droplets 2 directly from the thin channels of the printhead nozzles 1.
Jet sputtering methods are described in more detail in [9–10]. Features of this method of 3D-MID technology are the need of using multiple heads in parallel, a special suspension system for the printhead or system of heads, the observance of a constant gap (2–3 mm) between the nozzle of the printhead and the substrate, the use of special types of fast-hardening viscous "inks" with high adhesion.
2.6. 3D photolithography
3D photolithography (3D Photoimaging) is one of the first methods for the structuring of 3D-MID products, as the traditional technology of photolithography has long been firmly established itself in the manufacturing of electronic components [11].
The basis of the classical process of photolithography is to create the necessary topology of the surface by light exposure (most often in UV range) of the photopolymer. Depending on the type of photopolymer (Fig.8), irradiation through a photomask stimulates the process of breaking photopolymer chains, that is "destruction" (positive photoresist) with subsequent removal of the irradiated parts of the substrate surface (Fig.8A), or polymerization, that is "hardening" (negative photoresist), followed by removing the unexposed part of the photopolymer from the surface of the substrate (Fig.8b).
Fig.9 shows a typical sequence of technological operations of 3D photolithography by a semi-additive metallization scheme, which repeats the operations of a semi-additive laser structuring to the point of overlay of the photomask.
The metallization process starts with the deposition of the thin conductive copper sublayer 2 (about 1 µm) over the entire free surface of the bulk substrate 1 (Fig.9a, b). A thin conductive sublayer may be formed by any method that meets the requirements of adhesive strength, conductivity and thickness, for example, by chemical deposition, vacuum deposition, thermal spray metallization, etc.
Further, on the conductive layer 2 a photoresist 3 is applied (Fig.9c), which is exposed directly by a laser or 3D photomask in accordance with a given topological pattern. Then the development of the photoresist (Fig.9d) forms a windows for the further galvanic deposition of a thick conductive layer 4 (Fig.9e). The thickness of the layer depends on the requirements. Currently the thickness of the deposited copper layer is about 25 to 35 µm. Over the formed structure another layer of photoresist 5 is applied (Fig.9f), which is a mask for etching the underlayer of copper 2 (Fig.9g). The final step is the removal of residual photoresist from the obtained conductive pattern (Fig.9j).
The key advantages of the technology:
•reduction of production costs, less undercut under the mask and most environmentally friendly than subtractive methods of etching of a "thick" foil;
•use of any solid not foiled materials as a substrate;
•high enough resolution (width of conductor is less than 100 µm);
•good reproducibility of the technology when forming the conductors with a thickness less than 0.1 mm.
The key disadvantages of the technology:
•necessity of application of different technologies of deposition and etching for various materials;
•difficulty of ensuring good adhesion of the underlayer of metal to some of the material of the substrate;
•the need for additional cleaning of products associated with the contamination of the ends of the substrate during removal of the photoresist or galvanic deposition of conductors.
3. Gas-phase (dry) 3D MID technology (DS-3D MID)
A large number of physical technologies of metal deposition do not use any aqueous solutions of metal salts containing the deposition material in the form of anions. These methods are called "dry" as they use exclusively gas-phase reactions under vacuum or at atmospheric pressure.
Vacuum ion-plasma treatment is a standard and reproducible ECB (Electronic Component Base) technologies. This is due to its high resolution, precise control of process variables, the ability to locally supply high energy (100 eV or more) and localise the effects in the surface layer of material (2–100 nm). Vacuum deposition methods can be divided into two groups:
•physical vapour deposition (PVD) methods, which include e.g. vacuum thermal evaporation (VTE), electron beam evaporation (EBE), cathode sputtering (CS) magnetron sputtering (MSpD) etc.;
•chemical vapour deposition (CVD) from the vapour phase with and without activation by plasma.
The main drawback of the existing ion-plasma methods from the point of view of economic efficiency can be called the need to create a reduced pressure up to 10–6 Torr. The means of obtaining and maintaining the vacuum account for a significant part of the value of modern microelectronic production and require significant expenditures associated with their operation. This circumstance sparked interest in the development of ion-plasma methods of material processing at atmospheric pressure as indicated by the constant appearance of messages on this topic in various scientific publications.
Currently the largest two gas phase (dry) S-3D MID technologies (hereinafter – DS-3D MID) are most commonly used:
•gas-plasma metallization (Flamecon);
•cold plasma deposition (Plasma dust).
3.1. Gas-plasma metallization
The peculiarity of this technology is the simultaneous effect on the applied material from the high-temperature heat source and the kinetic energy of the gas stream [7]. The flame spraying method comprises three successive stages:
•melting by high temperature flame (e.g. oxy-acetylene) of the material fed into the combustion chamber in the form of a wire with the diameter of 1–3 mm (in the deposition of metal) or powder (in the deposition of dielectrics) and its partial evaporation in the form of microdroplets;
•transfer of metal particles on the preformed surface of a part with compressed air or an inert gas;
•formation of coatings in the collision of "cold" particles accelerated by a supersonic gas flow, with the part surface.
The flame metallization system is shown in Fig.10. The transport gas flow while moving along the spray nozzle 2 sucks metal particles into it out of the powder container 1. Then, these particles are melted by the heat released during the combustion of the gas mixture also supplied to the spray nozzle through the ignition system 6. The deposited powder material is blown into a special flame burner, wherein the particles are heated to a very plastic or molten state. The resulting deposited metal melt is ejected from the spray nozzle as a focused spray jet 3. The deposited metal particles contained in the jet by striking the surface of the substrate 5 engage with it and form a conductive layer 4. In a single pass coating is from 20 to 100 µm thick is created.
The key advantages of the technology:
•spraying heterogeneous metal (aluminum, copper, zinc, tin, nickel etc.) and polymeric materials;
•possible applying coatings to a variety of products in terms of size;
•relatively low temperatures of the substrate surface (10 to 150°C).
The key disadvantages of the technology:
•use of gases including explosive ones (compressed air, acetylene, oxygen);
•low deposition efficiency (2–4 kg/h);
•high porosity of the applied metal coating;
•significant (50%) losses of the sprayed material;
•low coating uniformity in terms of thickness.
3.2. Cold plasma deposition
Arc is one of the most common types of self-electric discharge in gas, in which the discharge phenomena are concentrated in a narrow and brightly glowing plasma column. The arc discharge is ignited by quite a strong electric field’s passing through the gas which is ionized under the influence of various factors. The arc discharge may be generated in various environments and at various pressures. It is known that with the increased pressure in the environment, the current strength in the arc increases, and the transverse dimensions of its cord decreases. A DC arc plasma torch is one of the most common designs of electric arc plasma torches operating at ambient (or slightly elevated) pressure [12].
The equipment consists of the following sub-systems:
•an electrical subsystem including main electrodes, auxiliary electrodes (discharge ignition), insulators and elements connecting to sources of supply;
•gas subsystem comprising plasma forming gas input channels, the mixture of the transport gas and powder of the material to be deposited, and, if necessary, the protective gas of the cathode or additional focusing gas of the nozzle as well as seals and devices for connecting to external systems;
•discharge chamber provided with a nozzle which gives the gas flow in the plasma torch the desired rate and direction of movement;
•system to move the plasma torch relative to the substrate surface in the horizontal direction to form a predetermined surface structure and in the vertical direction for adjusting the deposition conditions. In some types of units, substrate is moved relative to the plasma torch nozzle.
One of the DC arc torch designs is shown in Fig.11. In this type of plasma torch, arc discharge is generated by the ignition system of discharge 4 between the inner heat-resistant electrode (cathode) 1 and a water-cooled nozzle (anode) 2. The interelectrode gap of the discharge chamber 3 is supplied with the pressurized plasma-supporting gas 6, for example, inert gas which blows the arc out of the opening in the form of a "plume" 9. In the interelectrode gap with plasma gas supplied is a mixture of the transport inert gas and the powder of the coating material 8. Under the influence of the high-density current melted is the powder of the coating material which is blown in the form of microdrops from the nozzle of the plasma torch 5. Since the "plume" of the plasma torch is a gas medium, the transport and plasma-supporting gases disperse with distance from the nozzle, and the melt microdrops continue to move in the direction of the substrate 10 of a free form. Since the main flow of the deposited material moves targetedly towards the substrate, formed on it is a controlled deposition zone, the so-called "melting zone" 11.
Coating is formed by molten particles that are ‘welded’ to the substrate surface and to each other. The coating quality depends strongly on the degree of heating and particle velocity on impact with the substrate, which is determined by the rate, temperature and plasma thermal conductivity at the outlet of the plasma torch, and the thermal properties of the sprayed material.
Fine metal powders (Cu, Sn, Zn, Ag, tungsten etc.), the oxides of aluminium, zirconium, silicon and titanium as well as carbides, borides, the nitrides of tantalum, silicon, niobium and hafnium as well as dielectrics (polymers) with a particle size from 100 nm to 20 µm are used as the deposited materials.
For the creation of plasma used are inert (argon, neon, helium) and active gases and compounds (nitrogen, oxygen, hydrogen, hydrocarbons, ammonia). The selected plasma gas and plasma temperature are determined by the chemical composition of the deposited material, coating quality requirements and cost of coating. Only inert gases are most often used as transport gases.
In some cases, the deposition material is in the form of wire, which is fed into the plasma jet, melted, atomised, and the droplets are accelerated and deposited on the sprayed surface. In this case, delivered to the wire is the anode potential which facilitates its melting and increases the overall efficiency of the process.
The key advantages of the technology:
•possibility of applying various materials (conductors, dielectrics, semiconductors) in the form of separate elements (copper, gold, silicon) as well as compounds (polymers, alloys, ceramics, composites);
•possibility of applying the compounds of varying stoichiometry;
•high adhesion of coatings;
•sufficiently high precision of application of material in terms of thickness.
The key disadvantages of the technology:
•complexity of the unit design;
•relatively low coating deposition rate;
•low resolution of the created designs;
•use of multiple types of gases.
To be continued in the next issue
2. Liquid S-3D MID
Liquid technologies are divided into the following types:
•two step or two shot molding;
•laser direct structuring (LDS);
•aerosol jet or maskless technology (mesoscale, material deposition – M3D);
•jet sputtering;
•3D photolithography.
2.1. Two step or two shot molding
In the two step molding two different types of polymers are used, one of which may be conductive and the second – dielectric. The first polymer is the main material of the hull structure (first injection), and the second polymer is a filler that selectively fills the free volume and forms the final shape of the part (second injection).
A quite large number of variations of this technology is known, however, the most widely it is developed by Kollmorgen (Germany) in the PCK- and PA-processes [1–4], which are "positive" and "negative" methods of the two step molding by analogy with the processes of conventional photolithography (Fig.1).
More expensive catalytic SKW-process [4] (Fig.2), which is a kind of PCK, is used much less.
As an example, consider the three stages of the PCK-process:
•the conductive substrate 1 (Fig.1a) is made of plastic with a metal filling (e.g., palladium particles) by a injection molding (first injection);
•the second (dielectric) layer of the polymer material 2 that filling the entire surface of the substrate, except areas for the future contact pads, is selectively applied to the surface of the substrate 1 by the second injection in a special mold. The material of the second injection, as a mask, generates the topology of the switching conductors and contact pads (Fig.1b);
•layers of different metals (copper, nickel, gold, etc.) are applied to all open areas of the conductive substrate of the first injection by galvanic (electrochemical) method. Thickness of layers 3 is adjusted by the deposition conditions (Fig.1c).
The key advantages of two step molding:
•structure of conductor paths is formed in the molding process;
•it is possible to apply layers of materials up to 0.5–5 µm;
•high performance (including the high growth rate of metal conductors).
The key disadvantages of the technology:
•the use of two injections of polymers (metallized and not metallized) causes a significant restrictions in the choice of materials, one of which is essentially a mask;
•low resolution (minimum to 0.25 mm, depending on the yield strength of the materials [4]);
•inability to make quick changes in the design of the conductive circuit;
•due to the high cost of the tools, the technology is efficient only under high volume production;
•use of environmentally harmful electrolytes.
2.2. Laser direct structuring
Laser direct structuring (LDS) is an additive method of applying 3D conductive circuits on plastic case [5]. In literature LDS is subdivided into composite and printing-additive.
The basic idea of composite technology of laser direct structuring is the introduction of special metal-containing organic additives in the polymer melt during molding. The additive is heat resistant and prevents the formation of active centers in the process of injection molding. It has a little effect on the basic properties of the plastic, as it contains inactive chemicals that are activated only by local high temperature heating by laser radiation and appear only during ablation of a solid surface. The thermal degradation under the influence of laser radiation on the surface of these composites leads to the evaporation or decomposition of the organic part of the polymer (formation of the rough surface) and to the connection of the individual metal particles in the microspheres, which later become the centers of crystallization of copper during its electrochemical deposition.
Thus, the composite LDS involves four basic operations: injection molding of the substrate (body) with special additives; laser structuring (laser processing of the surface of the substrate with the formation of active centers); metallization (galvanic electrochemical deposition of copper) of modified plastic; the application of the top (protective) layer (except areas of contact pads).
Fig.3. shows the scheme of device for composite laser structuring. The initial plastic substrate 5 is subjected to laser structuring. The rotary mirrors 3 are used for deflection of the laser beam 2. As a result of processing the modified plastic is divided into a metal and organic residual groups forming an activated areas 4. These areas in the future become a center of crystallization during chemical deposition of conductive layers 6 (copper, nickel, gold, etc.) with a total thickness of 10–20 µm.
Printing-additive LDS (Pro Paint LDS) is largely similar to composite with two main differences: first, no metal-containing organic additives in the polymer substrate ("unprepared" plastics are used); second, after the laser structuring of the surface a special conductive "ink" is applied.
Printing-additive LDS consists of six main operations (Fig.4):
•injection molding of the substrate (case);
•laser structuring (processing of the substrate surface by laser light);
•jet-spray application of a conductive "ink", which is a two-component system of "primer" and hardening agent;
•the removal (washing) of the excess "ink" from unstructured surfaces;
•metallization (galvanic electrochemical deposition of copper) of modified plastic;
•application of the top (protective) layer (except areas of contact pads).
For printing – typically the jet-spray application of a conductive "ink" – is used standard sprayer or specialized system (see 2.3). The layer of "ink" with a thickness of about 30 to 40 µm is applied in two stages, and then hardens in the air or using additional heat exposure.
Printing-additive LDS is more labour-intensive and primarily intended for prototyping, since the cost and properties of such products for continued use are not comparable with the properties of composite plastics [6].
The key advantages of LDS:
•only one material is used for forming the substrate (in contrast to two-stage molding);
•flexibility of the process allows to make changes in the design of conductive circuit by reprogramming of the laser;
•resolution to 80 µm, with the possibility of coating in a fairly remote places of the substrate;
•no need to use masks;
•sufficient productivity at relatively low cost of equipment.
Composite technology is also characterized by high adhesion of the deposited metal to the substrate caused by the formation of microrelief in the process of laser activation, and printing-additive technology – by the use of almost any polymer materials and products based on them.
The key disadvantages of the LDS:
•the complexity of the formation of thick layers of functional materials;
•possible thermal damage to the substrate during its laser activation;
•use of environmentally harmful electrolytes.
Additional disadvantage of composite LDS is the limited choice of materials because of the need for the use of special polymers or additives in the manufacture of the substrate, and disadvantage of printing-additive technology is a low adhesion of the conductive "ink" to the developed microrelief obtained by laser activation of the surface, and the presence of additional operations, which significantly increases the cost of manufacturing process.
2.3. Laser subtractive structuring
Laser subtractive structuring (LSS) is based on one of the first, and, consequently, proven technologies of production of circuit boards consisting of a dielectric substrate and foil-coated metal material (mostly copper). A distinctive feature of the LSS from traditional subtractive technology is the impossibility of forming a conductive thick film coating by hot stamping, as on the 2D surface.
Laser subtractive structuring comprises the following steps:
•single- or multicomponent injection molding of the substrate (case);
•deposition of a sublayer of copper (chemical, plasma-chemical, vacuum, thermal, etc.) (Fig.5b). If only chemical deposition of an copper underlayer is used, then before metallization it is advisable to activate the required surface area of the substrate, for example by laser structuring (Fig.5a);
•electrochemical deposition of copper on the metallized surface of the sublayer (Fig.5c);
•deposition of mask (a thin layer of plating resist). A dielectric or conductive material with a high degree of etching selectivity relative to the layer of copper can be used as plating resist (Fig.5d);
•laser structuring (burning) of a thin layer of plating resist (Fig.5e);
•chemical or plasma-chemical etching of copper (Fig.5f);
•if necessary (depending on the operating conditions of the products or other requirements), the removal of residual plating resist (Fig.5g).
2.4. Aerosol-jet deposition
Aerosol-jet deposition [7–8] provides the direct application of conductive films of different liquid materials (suspensions) to the surface of finished 3D parts produced, for example, by injection molding. The suspension includes a solvent, stabilizers, modifiers, polymers, solid particles, etc., and are divided into suspensions of conductive materials, suspensions of dielectric materials and suspensions of semiconductor materials, therefore, the diameter of solid particles may vary considerably depending on the type of deposited material and generally is in the range of 0.05–1 µm.
The device for of aerosol-jet deposition (Fig.6) is based on the special two-section sprayer. In the first section of sprayer the prepared suspension 2 is placed in the evaporator (heater) 1, via which the stream of transport inert gas is passed, creating an aerosol 3 (suspended droplets). Large drops (5 µm or more) return under the action of gravity back into the tank.
The second section of the sprayer is a dual spray nozzle 4. The protective (focusing) gas, which forms the focusing ring of the aerodynamic flow, is supplied under pressure to the outer contour of the spray nozzle. This flow creates in the inner contour of the spray nozzle low pressure, which directs the aerosol due to the pressure difference. The annular aerodynamic flow focusing a stream of aerosol into a narrow beam (flux) with a diameter of 5 to 10 µm, which gives the opportunity to apply the material 6 on the hard-to-reach surfaces 7 with high accuracy and without the use of masks. Distance from spray nozzle to the deposited surface can be up to 5–7 mm.
After application to a substrate, the suspension is cured during the process of sintering or polymerization in the standard drying ovens, ovens for reflow soldering, by laser radiation, LEDs, etc.
The application of aerosol is carried out either by moving the nozzle relative to the substrate or by moving the substrate relative to the nozzle. To obtain separate structures (tracks, pads, or elements of TEU) flow of aerosol can be damped by special device.
The key advantages of the technology:
•only one material is used in the process of forming the substrate;
•it is possible to apply thin layers (10–20 nm) of materials with a resolution down to 5 µm;
•avoidance of thermal damage of the substrate;
•flexibility of the process that allows to quickly make changes to the pattern of the conductive circuits;
•the possibility of obtaining conductive, semiconducting, dielectric, and polymer (PTF, polymer precision thick-film) coatings on virtually all substrate materials;
•no need to use masks;
•possibility of applying coatings in hard-to-reach areas of the substrate;
•high performance at a relatively low cost of processing equipment.
The key disadvantages of the technology:
•use of two gases (transport and focusing);
•need for careful selection of the suspensions for various materials of the substrate to ensure good adhesion of the deposited material;
•need for the curing.
2.5. Jet sputtering (metallization)
Jet sputtering is known for a long time and currently it is widely used in printing, in the production of packaging materials, for applying a photoresist in microelectronics, in printed electronics, etc.
At jet metallization, the scheme of which is shown in Fig.7, the applied material is deposited on free surfaces of the 3D substrate 3 in the form of droplets 2 directly from the thin channels of the printhead nozzles 1.
Jet sputtering methods are described in more detail in [9–10]. Features of this method of 3D-MID technology are the need of using multiple heads in parallel, a special suspension system for the printhead or system of heads, the observance of a constant gap (2–3 mm) between the nozzle of the printhead and the substrate, the use of special types of fast-hardening viscous "inks" with high adhesion.
2.6. 3D photolithography
3D photolithography (3D Photoimaging) is one of the first methods for the structuring of 3D-MID products, as the traditional technology of photolithography has long been firmly established itself in the manufacturing of electronic components [11].
The basis of the classical process of photolithography is to create the necessary topology of the surface by light exposure (most often in UV range) of the photopolymer. Depending on the type of photopolymer (Fig.8), irradiation through a photomask stimulates the process of breaking photopolymer chains, that is "destruction" (positive photoresist) with subsequent removal of the irradiated parts of the substrate surface (Fig.8A), or polymerization, that is "hardening" (negative photoresist), followed by removing the unexposed part of the photopolymer from the surface of the substrate (Fig.8b).
Fig.9 shows a typical sequence of technological operations of 3D photolithography by a semi-additive metallization scheme, which repeats the operations of a semi-additive laser structuring to the point of overlay of the photomask.
The metallization process starts with the deposition of the thin conductive copper sublayer 2 (about 1 µm) over the entire free surface of the bulk substrate 1 (Fig.9a, b). A thin conductive sublayer may be formed by any method that meets the requirements of adhesive strength, conductivity and thickness, for example, by chemical deposition, vacuum deposition, thermal spray metallization, etc.
Further, on the conductive layer 2 a photoresist 3 is applied (Fig.9c), which is exposed directly by a laser or 3D photomask in accordance with a given topological pattern. Then the development of the photoresist (Fig.9d) forms a windows for the further galvanic deposition of a thick conductive layer 4 (Fig.9e). The thickness of the layer depends on the requirements. Currently the thickness of the deposited copper layer is about 25 to 35 µm. Over the formed structure another layer of photoresist 5 is applied (Fig.9f), which is a mask for etching the underlayer of copper 2 (Fig.9g). The final step is the removal of residual photoresist from the obtained conductive pattern (Fig.9j).
The key advantages of the technology:
•reduction of production costs, less undercut under the mask and most environmentally friendly than subtractive methods of etching of a "thick" foil;
•use of any solid not foiled materials as a substrate;
•high enough resolution (width of conductor is less than 100 µm);
•good reproducibility of the technology when forming the conductors with a thickness less than 0.1 mm.
The key disadvantages of the technology:
•necessity of application of different technologies of deposition and etching for various materials;
•difficulty of ensuring good adhesion of the underlayer of metal to some of the material of the substrate;
•the need for additional cleaning of products associated with the contamination of the ends of the substrate during removal of the photoresist or galvanic deposition of conductors.
3. Gas-phase (dry) 3D MID technology (DS-3D MID)
A large number of physical technologies of metal deposition do not use any aqueous solutions of metal salts containing the deposition material in the form of anions. These methods are called "dry" as they use exclusively gas-phase reactions under vacuum or at atmospheric pressure.
Vacuum ion-plasma treatment is a standard and reproducible ECB (Electronic Component Base) technologies. This is due to its high resolution, precise control of process variables, the ability to locally supply high energy (100 eV or more) and localise the effects in the surface layer of material (2–100 nm). Vacuum deposition methods can be divided into two groups:
•physical vapour deposition (PVD) methods, which include e.g. vacuum thermal evaporation (VTE), electron beam evaporation (EBE), cathode sputtering (CS) magnetron sputtering (MSpD) etc.;
•chemical vapour deposition (CVD) from the vapour phase with and without activation by plasma.
The main drawback of the existing ion-plasma methods from the point of view of economic efficiency can be called the need to create a reduced pressure up to 10–6 Torr. The means of obtaining and maintaining the vacuum account for a significant part of the value of modern microelectronic production and require significant expenditures associated with their operation. This circumstance sparked interest in the development of ion-plasma methods of material processing at atmospheric pressure as indicated by the constant appearance of messages on this topic in various scientific publications.
Currently the largest two gas phase (dry) S-3D MID technologies (hereinafter – DS-3D MID) are most commonly used:
•gas-plasma metallization (Flamecon);
•cold plasma deposition (Plasma dust).
3.1. Gas-plasma metallization
The peculiarity of this technology is the simultaneous effect on the applied material from the high-temperature heat source and the kinetic energy of the gas stream [7]. The flame spraying method comprises three successive stages:
•melting by high temperature flame (e.g. oxy-acetylene) of the material fed into the combustion chamber in the form of a wire with the diameter of 1–3 mm (in the deposition of metal) or powder (in the deposition of dielectrics) and its partial evaporation in the form of microdroplets;
•transfer of metal particles on the preformed surface of a part with compressed air or an inert gas;
•formation of coatings in the collision of "cold" particles accelerated by a supersonic gas flow, with the part surface.
The flame metallization system is shown in Fig.10. The transport gas flow while moving along the spray nozzle 2 sucks metal particles into it out of the powder container 1. Then, these particles are melted by the heat released during the combustion of the gas mixture also supplied to the spray nozzle through the ignition system 6. The deposited powder material is blown into a special flame burner, wherein the particles are heated to a very plastic or molten state. The resulting deposited metal melt is ejected from the spray nozzle as a focused spray jet 3. The deposited metal particles contained in the jet by striking the surface of the substrate 5 engage with it and form a conductive layer 4. In a single pass coating is from 20 to 100 µm thick is created.
The key advantages of the technology:
•spraying heterogeneous metal (aluminum, copper, zinc, tin, nickel etc.) and polymeric materials;
•possible applying coatings to a variety of products in terms of size;
•relatively low temperatures of the substrate surface (10 to 150°C).
The key disadvantages of the technology:
•use of gases including explosive ones (compressed air, acetylene, oxygen);
•low deposition efficiency (2–4 kg/h);
•high porosity of the applied metal coating;
•significant (50%) losses of the sprayed material;
•low coating uniformity in terms of thickness.
3.2. Cold plasma deposition
Arc is one of the most common types of self-electric discharge in gas, in which the discharge phenomena are concentrated in a narrow and brightly glowing plasma column. The arc discharge is ignited by quite a strong electric field’s passing through the gas which is ionized under the influence of various factors. The arc discharge may be generated in various environments and at various pressures. It is known that with the increased pressure in the environment, the current strength in the arc increases, and the transverse dimensions of its cord decreases. A DC arc plasma torch is one of the most common designs of electric arc plasma torches operating at ambient (or slightly elevated) pressure [12].
The equipment consists of the following sub-systems:
•an electrical subsystem including main electrodes, auxiliary electrodes (discharge ignition), insulators and elements connecting to sources of supply;
•gas subsystem comprising plasma forming gas input channels, the mixture of the transport gas and powder of the material to be deposited, and, if necessary, the protective gas of the cathode or additional focusing gas of the nozzle as well as seals and devices for connecting to external systems;
•discharge chamber provided with a nozzle which gives the gas flow in the plasma torch the desired rate and direction of movement;
•system to move the plasma torch relative to the substrate surface in the horizontal direction to form a predetermined surface structure and in the vertical direction for adjusting the deposition conditions. In some types of units, substrate is moved relative to the plasma torch nozzle.
One of the DC arc torch designs is shown in Fig.11. In this type of plasma torch, arc discharge is generated by the ignition system of discharge 4 between the inner heat-resistant electrode (cathode) 1 and a water-cooled nozzle (anode) 2. The interelectrode gap of the discharge chamber 3 is supplied with the pressurized plasma-supporting gas 6, for example, inert gas which blows the arc out of the opening in the form of a "plume" 9. In the interelectrode gap with plasma gas supplied is a mixture of the transport inert gas and the powder of the coating material 8. Under the influence of the high-density current melted is the powder of the coating material which is blown in the form of microdrops from the nozzle of the plasma torch 5. Since the "plume" of the plasma torch is a gas medium, the transport and plasma-supporting gases disperse with distance from the nozzle, and the melt microdrops continue to move in the direction of the substrate 10 of a free form. Since the main flow of the deposited material moves targetedly towards the substrate, formed on it is a controlled deposition zone, the so-called "melting zone" 11.
Coating is formed by molten particles that are ‘welded’ to the substrate surface and to each other. The coating quality depends strongly on the degree of heating and particle velocity on impact with the substrate, which is determined by the rate, temperature and plasma thermal conductivity at the outlet of the plasma torch, and the thermal properties of the sprayed material.
Fine metal powders (Cu, Sn, Zn, Ag, tungsten etc.), the oxides of aluminium, zirconium, silicon and titanium as well as carbides, borides, the nitrides of tantalum, silicon, niobium and hafnium as well as dielectrics (polymers) with a particle size from 100 nm to 20 µm are used as the deposited materials.
For the creation of plasma used are inert (argon, neon, helium) and active gases and compounds (nitrogen, oxygen, hydrogen, hydrocarbons, ammonia). The selected plasma gas and plasma temperature are determined by the chemical composition of the deposited material, coating quality requirements and cost of coating. Only inert gases are most often used as transport gases.
In some cases, the deposition material is in the form of wire, which is fed into the plasma jet, melted, atomised, and the droplets are accelerated and deposited on the sprayed surface. In this case, delivered to the wire is the anode potential which facilitates its melting and increases the overall efficiency of the process.
The key advantages of the technology:
•possibility of applying various materials (conductors, dielectrics, semiconductors) in the form of separate elements (copper, gold, silicon) as well as compounds (polymers, alloys, ceramics, composites);
•possibility of applying the compounds of varying stoichiometry;
•high adhesion of coatings;
•sufficiently high precision of application of material in terms of thickness.
The key disadvantages of the technology:
•complexity of the unit design;
•relatively low coating deposition rate;
•low resolution of the created designs;
•use of multiple types of gases.
To be continued in the next issue
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